Many types of databases employ a schema to map data content to data attributes. In a relational database, the schema may group a collection data attributes and their corresponding data items into a logical entity known as a table. Each data attribute is referred to as a “column” and each unique set of related data items is referred to as a “row.” Each row in the table is uniquely identified by a special column, a primary key. By understanding the description of the schema (e.g. the tables, columns, etc.) a database user can retrieve data by referring to the data attributes without the knowledge of the actual data content.